Paralleling lithium

I have a possible project which would probably require paralleling a lot of individual lithium laptop cells, like 9x 1.6ah unprotected cells in parallel and then putting 8 of these banks in series to meet the voltage requirements. This is necessary not only to increase the overall AH of the bank but to increase the current capabilities. Don't have a clear spec on how much current is required but it's probably around 6A average, 12A peak, far more than a 1C avg or 2C peak discharge for each cell.

I would be able to build a regulator to clamp each cell at 4.2V during charging. I'd feed the string a constant current at first and after the first bank hits 4.2V I'd fold back the current to keep the shunt reg's heat generation "reasonable", then shut off all the charging current when the last series bank hits 4.2V. I will also have the pack designed to shut off when the any of the parallel banks drops to 2.75V.

There is no other cell type I'd be able to use and stay within budget. Nor will it be practical or cost-effective to separate out these cells for charging independently.

I've seen laptop packs may parallel 2 or 3 cells inside. I know NiMH for example does not charge or even store correctly in parallel, one fully charged NiMH may take current from another one in parallel leading to one overcharged cell and one partially run-down one.

So my question is this. Are there any "gotchas" I'm missing regarding paralleling greater numbers of lithium laptop cells, such as parallel balancing issues, that become problematic when attempting to use many cells paralleled together with no way to isolate them? Another hypothetical problem would be cells of different impedance in a bank resulting in some cells bearing more current and discharging faster than others, yet this doesn't appear to be a real problem because if one starts to discharge more the open cell voltage drops would would soon shift current over to the higher resistance cells resulting in an even discharge.

Hello Oznog,

There are no problems charging or discharging cells in parallel.

There are no problems discharging cells in series or in parallel.

There can be a problem charging cells, or banks of cells, in series. In this case you need to be able to balance each cell, or bank of cells, in the series string to insure that it does not exceed the maximum voltage of the cell.

For example... If you have 3 cells in series, your charger would charge them up to a maximum voltage of 12.6 volts.

If all 3 cells are at 4.2 volts, you are balanced, however if the voltages of your cells were 4.1, 4.0, and 4.5, you would probably have a "vent with flame" issue with the cell at 4.5 volts.

On the other hand, if your battery pack is used in an application where they last 2 hours or longer, are charged at a rate that completes the charge in about 3 hours, if the low voltage cutoff is limited to around 3.0 - 3.2 volts per cell, and if you are using premium cells (Sanyo, Sony, Panasonic, LG), you can frequently get away with not balancing over 400 - 500 charge/discharge cycles. After that, the pack imbalance starts to grow to a point where it can become dangerous.

Tom

I so not like the idea of series/parallel battery configuration. They will discharge amongst themselves. Unless you can switch the parallels on and off.

Hello Fxstsb,

With Li-Ion chemistry, there is no problems with parallel set ups. The problem comes with series charging.

Tom

SilverFox said:

Hello Fxstsb,

With Li-Ion chemistry, there is no problems with parallel set ups. The problem comes with series charging.

Tom

Click to expand...

Paralleling Li cells is standard practice in the RC world.
The important thing is to keep the cells' voltages equal if they are not always in one pack ie don't disconnect two cells, discharge one then parallel it with the first again. This will cause problems, maybe fire or explosion. Instead recharge the discharged cell until it matches the voltage of the first.
I always keep my cells together, or recharge two packs to within a millivolt or so before connecting them up. Never had a problem doing it that way.
Oh and remember to use series charging measurement to balance the cells voltages as they charge.

I use this type of circuit when building high voltage Li-Ions battery packs.

It prevents mishaps due to cell imbalance especially during charging, as Tom pointed out:

http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=3248

Best Regards

Anthony

SilverFox said:

Hello Fxstsb,

With Li-Ion chemistry, there is no problems with parallel set ups. The problem comes with series charging.

Tom

Click to expand...

Thank You.